FIG. 1 shows a typical wireless system infrastructure, comprising a set of access points (APs), also referred to as base stations (BS), each connected to a wired network through what is referred to as a backhaul link. The wireless links exist between the APs and the user stations (STAs). In some scenarios, the cost of connecting a given AP directly to the wired network makes an alternative option more attractive, which is to connect the AP indirectly to the wired network via wireless connections to its neighboring APs. This is referred to as a Mesh architecture. FIG. 2 shows a block diagram of a simple Mesh architecture comprising a plurality of Mesh points (MPs), each capable of supporting control, management and operation services for the Mesh. The MPs may either be a dedicated infrastructure device (e.g., a Mesh AP (MAP)) or a user device (e.g., a STA) that is able to fully participate in the formation and operation of the Mesh network. Advantages of using a Mesh infrastructure include ease-of-use and speed of deployment since a radio network can be deployed without having to provide backhaul links and interconnection modules for each AP.
One very important operational consideration is that Tx power settings of Mesh nodes are regulated in order to meet regulatory requirements. Operation of wireless radio communications today is regulated by the FCC (and their counterparts in other countries). In particular, certain maximum Tx power settings are mandated in order to minimize interference of un-licensed radio equipment such as WLANs for most frequency bands. Moreover, these regulatory requirements usually change per regulatory domain (e.g., U.S., Europe, Japan). Typical regulatory requirements for conventional WLANs operating in infrastructure mode (basic service set (BSS)) or AdHoc mode (Independent BSS (IBSS)) are summarized as follows (i.e., Mesh operation is not addressed by this existing standard).
Transmit power control (TPC) under IEEE 802.11h for 5 GHz band WLANs is primarily motivated by different regulatory Tx power allowances in the 5 GHz band assignments in Europe, but is also required by the FCC in the US. Different regulatory power requirements for the 5 GHz band include:                Lower U-NII (5.25-5.35 GHz, 4 channels) 40 mW US, 200 mW Europe        Middle U-NII (5.35-5.45 GHz, 4 channels) 200 mW US and Europe        (5.47-5.725 GHz, 11 channels) Europe-only, 1000 mW        Upper-U-NII (5.725-5.825 GHz, 5 channels) US-only, 800 mW        
The maximum admissible Tx power setting for any STA in the BSS or IBSS is the Power Constraint information element (IE) subtracted from the Regulatory Max Power value contained in the Country (IE). The Country IE (802.11d) is contained in BEACON and PROBE RESPONSE frames. Similarly, 802.11h puts the Power Constraint IE into BEACON and PROBES RESPONSE frames.
TPC under IEEE 802.11h adds a Power Capability IE to ASSOCIATION REQUESTS (RE-ASSOCIATION REQUESTS) sent from the STA to the AP (or STA in IBSS). This Power Capability IE is an indication of the possible minimum and maximum Tx power settings of the transmitting STA to the receiving STA.
Association attempts by STAs are to be refused by the AP or other STAs in an IBSS if the range indicated in their Power Capability IE does not allow operation with the current BSS regulatory settings. The AP is the only authority in the BSS that can change the admissible power setting for the BSS. In an IBSS, the STA that starts the IBSS is the one that sets admissible power settings and other STAs that subsequently broadcast the BEACON frame are required to propagate this initial power setting.
In the BSS case, the admissible power settings (regulatory in Country IE and offset in Power Constraint IE) can change during the lifetime of the BSS. Range control and interference reduction are specifically cited in 802.11h as one purpose for this feature. However, it is preferred that these changes in the settings should not happen “too often”.
One of the problems is that even if every BEACON can be used to change the power settings, not all STAs (for example the ones in packet switched (PS) mode) listen to every BEACON frame. Therefore, maximum Tx power changes are semi-static in the sense that it requires at least several target beacon transmission times (TBTTs) (hundreds of milliseconds) to have a new Tx power setting adopted by all STAs in the BSS.
Officially, 802.11h TPC requires a STA to check the admissible Tx power setting any time it tires to access the channel. However, it is doubtful that all manufacturers have implemented an automatic update from the latest received BEACON frame into their MAC firmware. It is reasonable to assume that this happens only once in a while, in extreme cases only during association or re-association.
TPC under 802.11h also introduces a TPC REQUEST/REPORT action frame pair. This TPC REQUEST action frame is used by a STA to request Tx power settings and link margin from another STA. The reported Tx power in the TPC REPORT action frame is the one used for sending the TPC report. The link margin reported is the one observed by the receiver when the TPC REQUEST action frame was received.
The IEs contained in the TPC REPORT action frame can also be put into the BEACON and PROBE RESPONSES, originally intended to address some special problems with IBSS mode. However, the link margin field in this case is meaningless and simply set to zero. These new 802.11h TPC-relevant IEs and action frames are found in Class 1 frames (i.e. they can be sent from and received by non-authenticated and non-associated STAs).
For completeness, 802.11h TPC functionality for the 5 GHz band is extended “as-is” into 2.4 GHz by the 802.11k draft amendment.
In order to allow ease of deployment and ease of adoption to a new deployment environment, a means to adopt allowed Tx power settings for Mesh equipment is needed. In addition to these regulatory considerations, adaptive Tx power levels are highly desirable to maintain high throughput and guaranteed QoS levels in a Mesh network.
The Tx and Rx power level settings of the participating nodes in a Mesh have a large impact on perceived communication and interference range. Perceived communication range is the distance over which a certain data rate can be sustained in a point-to-point or point-to-multipoint transmission). The perceived interference range is the distance over which a transmission can still disturb or degrade other ongoing transmissions from other nodes in the Mesh on a channel (or even on adjacent channels), even though the transmission itself cannot be reliably decoded any more.
Usually, the least possible Tx power setting in an MP conditioned on maintaining a given sustained data rate for a given Mesh link is the best approach to minimize co-channel and adjacent channel interference to other nodes in the Mesh. On the other hand, maximum possible Tx power level settings allow higher net data transmit rates because this directly impacts the SNR as seen by the intended receiver. This implies that MPs face conflicting needs and preferences in terms of which Tx and Rx power level settings to use. The ideal Tx power level setting for a particular MP is therefore a trade-off between maximizing individual data rates on particular links (higher data rates with higher Tx power settings) and maximizing overall Mesh performance (better performance with less interference and more spatial reuse on the same channel).
Rx power level settings, such as clear channel assessment (CCA) detection thresholds and minimum Rx sensitivity, impact the link budget and, as such, the SNR observed in the receiver. The Rx power level settings also impact the likelihood of failed channel access or collisions in carrier sense multiple access (CSMA)-based schemes such as 802.11 WLANs.
However, the level of interference perceived by the different nodes of a wireless Mesh system can vary widely both geographically and in time. This is because of the dynamic radio environment and real-time time-varying characteristics of transmissions in a Mesh, such as load per link or path, occupied channel time, etc.
Therefore, a means for dynamically controlling Tx and Rx power levels of Mesh nodes during the Mesh network lifetime is desirable in order to keep Mesh throughput and QoS high and at guaranteed levels. Also, channel changes motivated because of regulatory requirements need to be addressed in a wireless Mesh network.
While traditional WLANs (802.11a,b,g,j,n) do not provide any means today to allow for an adoption of Tx power settings other than at initial start-up, an amendment (802.11h) was made to Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications in order to satisfy regulatory requirements for operation in the 5 GHz band in Europe. IEEE 802.11h TPC only allows WLAN systems in the 5 GHz band to set Tx power settings during the initial association of incoming STAs and to some extent during the lifetime of the WLAN network (Infrastructure mode or AdHoc mode). However, the 802.11h amendment does not address the specific needs and constraints of Mesh systems. This case was simply not foreseen.
In particular, no means exist to ensure a selective Tx power change of a particular link within a Mesh. Moreover, only maximum admissible Tx power settings can be communicated. However, just as important as maximum admissible Tx power settings are, so too are the minimum power settings in order to guarantee establishment of links and to minimize probability of channel access collisions.
Variable Tx power settings would improve the radio efficiency of Mesh networks, but a method for achieving this feature is not provided by existing technology. Furthermore, a method for Tx power control needs to be devised to allow Mesh networks to meet certain regulatory requirements in the sense of 802.11h TPC similar to WLANs today operating in legacy infrastructure (such as in a BSS case) and AdHoc mode (such as in an IBSS case).